Conventional electronic assemblies include surface mount devices that may be transistors, amplifiers, or the like. These devices in operation create heat that must be removed from the surface of the device to avoid overheating and damage to the components. Prior art methods for removing heat from power devices include drilling thermal via holes through the bond pad and circuit board and metallizing these holes to provide a thermal path to the opposite side (or bottom side) of the board. These vias must be filled, typically with epoxy material, to prevent flux (which results in corrosion) from flowing through the holes to the opposite side of the circuit board. The heat removed to the bottom side of the circuit board must then be transported to a heat sink especially designed to dissipate the heat to the surrounding environment.
All the power devices that are thermally connected to the heat sink must be electrically isolated. Typically, this is accomplished by attaching the heat sink to the circuit board metallizations on the bottom side of the circuit board via a thermally conductive, electrically insulative adhesive. The adhesive layer contributes significantly to the overall thermal resistance necessitating a larger heat sink and limiting the heat removal capacity of the thermal stack. However, this process of removing heat from the power devices is costly due to the complex procedure of drilling metallizing and filling vias.
Other prior art methods for removing heat from power devices utilize, etched tri-metal (ETM) structures. In such ETM based systems, etched pedestals are created from ETM structures and serve the function of the filled vias that connect one side of a circuit board to the other side. More specifically, heat from power components passes through the entire copper backed ETM stack before being dissipated through a heat sink on the opposite side of the circuit board. For example, the heat sink may be a metal cross-car beam that supports an instrument panel in a vehicle. This construction provides a thorough path while electrically isolating the power components. Advantageously, such constructions create a thermal path through the substrate that is unimpeded by poorly conducting material. For example, a solder connection could be established between the top side circuit (onto which the power device is soldered) and the bottom side circuit. Alternatively, two solder connections to a core layer, one with the top side circuit and the other with the bottom side circuit could form a pass through with no poorly conducting materials in the thermal path. If a core pedestal is used, the pedestal may be electrically isolated from the ground by creating an island in the core, as disclosed in the above references. If a solder slug is used to connect the top and bottom layers, a hole in the ground plane may be drilled or etched to a larger diameter than the ETM layers ensuring that the solder does not wet the core.
While the above-mentioned prior art systems and a method do indeed reduce the resistance of the thermal stack upstream of the adhesive layer that bonds the circuit to the heat sink, increased heat transfer rates would be desirable. All of the heat transfer within the substrate is accomplished through heat conduction. One known problem that the prior art does not address is that heated air, in some instances, is trapped against the substrate and results in a reduced heat transfer efficiency.
Therefore, there is a need to improve the heat transfer rates through and around electronic circuit board and substrate that is absorbing and transferring heat from power devices. Therefore, the new and improved system and method for transferring heat from the circuit board should increase the rate of heat transfer from the thermal stack.